Hydrogen Chloride on Mars Hints at Continuing Volcanic Activity

The team states its central question directly: “The topic addressed here is is volcanism still contributing to this cycle on Mars today?” That framing keeps the conclusion open: the study tests a possible source rather than documenting an observed eruption.

What did the images show?

The researchers examined high-resolution images beneath the detection sites from three cameras: HiRISE, CaSSIS and CTX. The images revealed narrow fissures, some only tens of centimeters wide.

In places, the cracks formed swarms tens of kilometers long along topographic ridges, or strips of raised and lowered land. The team said researchers had not previously identified these features on Mars. They align with large-scale tectonic and volcanic patterns around Alba Mons.

Tectonic patterns describe deformation of a planet’s crust, while volcanic patterns trace activity involving magma. The researchers do not report an observed eruption at the fractures.

How could the gas reach the atmosphere?

The fractures could act as pathways for HCl to rise from bodies of magma at depth. After its upward journey, the gas may also encounter ice-rich permafrost, frozen ground laid down over long periods. The system need not produce a significant thermal signature, meaning a detectable heat pattern, which could explain why heat-based searches have found no active volcanism.

How much gas could be escaping?

The seepage estimate assumes hydrogen chloride remains in the Martian atmosphere for one sol, a single Martian day. Under that assumption, the release rate is less than 3 kilograms per second. The authors describe this as comparable to the low end of terrestrial volcanic fluxes, which range from a few to more than 100 kilograms per second.

The estimate therefore remains conditional on that atmospheric lifetime, rather than a direct measurement of gas emerging from the ground. The authors present it as a possible volcanic seepage estimate.

Could the seasonal peak also be volcanic?

The much larger seasonal release of HCl near perihelion, when Mars is closest to the Sun, is estimated at around 180 kilograms per second. The researchers do not think that this seasonal release comes directly from volcanism.

Some chlorine could still have an igneous origin, cycling between the atmosphere and surface reservoirs including perchlorates, chlorine-bearing salts and HCl-bearing ice. Perchlorates are a class of salts, while HCl-bearing ice contains hydrogen chloride.

“Overall, the presence and variability of hydrogen chloride in the Martian atmosphere appear to be governed by an interplay between episodic volcanic activity, atmospheric processing, and long-term sequestration into surface salts, perchlorates, and hydrogen chloride-bearing ices,” they said. Here, sequestration means storage in surface materials rather than continued release into the atmosphere.

The study treats episodic volcanism, atmospheric processing and long-term storage as linked parts of one chlorine cycle. Trokhimovskiy and colleagues present the detection as a potential signature, not direct proof of an active eruption.

The paper, “Enigmatic HCl detections as a potential signature of igneous activity on Mars,” appears in Science Advances, volume 12, issue 45, in 2026. The DOI is 10.1126/sciadv.aeb9422, and Alexander Trokhimovskiy is the first-named author.